When there is not enough oxygen for cells to use cellular respiration<span />
Explanation:
<u>the Paleozoic Era </u>
The Precambrian era describes a period of time that pre-dates Cambrian. This time typically references the entire period of the history of earth before the formation of rocks containing identifiable fossils occurred. It covers the majority of Earth’s history and is divided into the Hadean, Archean and the Proterozoic era.
Just before the Cambrian eruption, in the Paleozoic Era the environment was oxygenated, culminating in an explosion of new life in the Proteozoic Eon (2500-541 million years ago) in the form of primitive blue-green algae named cyanobacteria and oxygen based species. Multicellular species followed later, along with structure at higher levels, and complex life. Oxygen levels grew gradually over this period of time, from low levels.
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B) sunlight
Sunlight supports life by providing food and warmth which is needed for the population of practically anything on earth to survive
